Recently, so-called sweet oil wells containing carbon dioxide (referred to as CO.sub.2 hereafter) have been exploited because of increasing energy demand and a shortage of high quality oil resources that can be easily exploited. In addition, exploitation of rather small-scale oil wells, which have a short production life up to about 10 years because of relatively small reserves, is increasing. When the production efficiency of an oil well decreases, deaired (degassed) seawater is injected into the pipe in order to recover the oil production efficiency.
In the situation as mentioned above, an oil well pipe having high corrosion resistance to both CO.sub.2 and seawater, which contains small amounts of dissolved oxygen of about 500 ppb, is required. The seawater containing a small amount of dissolved oxygen as mentioned above, is referred to as "seawater" in this specification.
Conventionally an inhibitor is used to suppress corrosion of carbon steel pipes, when the pipe is used for both oil production and seawater injection. The inhibitor, however, not only increases production cost but also induces pollution. Therefore, there is a need in the art for an oil well pipe of steel which has sufficient corrosion resistance to eliminate the inhibitor.
It is known from the publications by A. Ikeda, M. Ueda and S. Mukai "Corrosion/83" NACE Houston, Paper No. 45, 1983, and Masakatsu Ueda and A. Ikeda "Corrosion/96" NACE Houston, Paper No. 13, 1996 that the corrosion rate of steel in CO.sub.2 environments decreases and resistance to general corrosion is improved, according to an increase of Cr content. In fact, the JIS SUS 410 series steels, which contain 12 to 13% of Cr ("%" for content of alloy elements means weight % in this specification) have already been utilized for oil well pipe.
However, the SUS 410 series steels are expensive because of the high Cr content thereof. In addition, such high Cr steels have a disadvantage in that they suffer localized corrosion (pitting) in seawater containing little dissolved oxygen.
A steel containing smaller amounts of Cr and cheaper than the 12 to 13% Cr steel is desired for an oil well pipe used for short life wells as described above. Furthermore, considering seawater injection, a steel having resistance to localized and general corrosion in seawater, i.e., a seawater resistant steel, is necessary.
Japanese Examined Patent Application 53-38687 discloses a low alloy seawater resistant steel containing 1.0 to 6.0% Cr and 0.1 to 3.0% Al. However, this steel is not for an oil well pipe, and the CO.sub.2 corrosion resistance thereof is not known.
Japanese Laid-Open Patent Publication No. 57-5846 discloses a steel containing 0.5-5% Cr and having resistance to sweet corrosion. While this reference states that such steel has good corrosion resistance in seawater containing CO.sub.2, the resistance is merely the general corrosion resistance, which has been estimated by corrosion weight loss. In addition, the microstructure thereof cannot be determined because the producing method of the steel is not disclosed.
Japanese Examined Patent Application No. 57-37667 proposes a wet CO.sub.2 resistant steel for line pipes, which contains more than 3.0% to 12.0% Cr. This steel's resistance against localized corrosion is improved in specific areas such as the welded portion, where the heat treatment history is different from other areas. The steel, however, cannot have a single phase martensite microstructure because of its low C content. Therefore, its tensile strength is low and its resistance to localized corrosion when used as a pipe is not sufficient.
Japanese Laid-Open Patent Publication No. 5-112844 discloses a steel pipe, which has good CO.sub.2 corrosion resistance and can be used for oil well pipes. However, the Cr content of this steel pipe is as low as 0.25-1.0%. Further, the pipe was not designed to improve the seawater corrosion resistance. In addition, the CO.sub.2 corrosion resistance of this pipe is improved mainly by a decarburized layer of more than 100 .mu.m thickness, which is formed in the inner surface of the pipe.
As mentioned above, it is already well known that increasing the Cr content improves the general corrosion resistance of the steel in CO.sub.2 environments. However, it is uneconomical to use steel having more than 10% Cr for short life oil wells such as 10 years or less. In addition, steel containing such a high content of Cr has the disadvantage of localized corrosion (pitting) in seawater of low dissolved oxygen. The oil well pipe becomes useless after suffering localized corrosion, which passes through the pipe wall, even if it has good general corrosion resistance. This means that not only general corrosion resistance but also localized corrosion resistance is remarkably important in a steel for an oil well pipe.